The present invention relates to honeycomb extrusion dies and more particularly to an improved extrusion die design offering improved mechanical durability.
The use of extrusion dies to form thin-walled honeycomb structures is well known. U.S. Pat. Nos. 3,790,654 and 3,905,743 to Bagley describe a basic design for such a die, that design incorporating a plurality of feedholes entering an inlet face of the die and extending through the body of the die to convey extrudable material to a discharge section formed on the die outlet surface by a criss-crossing array of interconnecting discharge slots. The discharge slots, which can be viewed as being formed by the gaps in a spaced array of parallel pins connecting with the die body, reform the extrudable material into an interconnecting wall structure for a channeled honeycomb body as that material is discharged from the discharge face of the die.
As the uses for such honeycomb structures have increased, so also has the need for extrusion dies capable of forming more finely structured honeycombs. A fundamental limitation of these dies, however, is the fact that neither the feedholes nor the discharge slots may be multiplied or reduced in size without limit, since the extrusion pressures used for plasticized powder extrusion require substantial stiffness or toughness in the die body and in the pin array to avoid die distortion or breakage.
Feedhole redesign is one area used in the past to achieve reductions in the required extrusion forces. U.S. Pat. Nos. 5,066,215 and 5,702,659 provide examples of such approaches, but both of these solutions involve impracticably high die fabrication costs and increased die fabrication complexity.
Another problem to be addressed at finer slot sizes is that of lateral batch flow redistribution. In the operation of these dies, longitudinal batch flow from the die body feedholes must rapidly transition to a combination of lateral and longitudinal flow within the die discharge section in order to adequately fill the discharge slot array. If the discharge slots are so narrow that lateral flow is non-uniform, defects such as marginal cell wall knitting, wavy or swollen cell walls, missing cell walls, and plugged cells will appear in the extruded product.
Approaches to improve the lateral redistribution of flowing honeycomb batch material within these dies have involved techniques such as the machining of lateral conduit channels behind the die discharge face. With similar results, pin arrays of smaller pin diameter have been used in combination with pin caps to provide both a fine discharge slot array and an opening of the spaces behind the discharge face where lateral batch redistribution can more readily occur.
U.S. Pat. Nos. 4,354,820 and 4,902,216 illustrate examples of die designs employing features of this type. In both of these cases, however, the reductions in pin cross-section incurred in the course of implementing these designs result in dies that are highly susceptible to damage via pin bending or pin detachment from impact or fatigue. Pin damage is not economically repairable with current technology.
The development of improved extrusion dies is somewhat impeded by the fact that all current economic methods for fabricating extrusion dies involve traditional rotating or straight line tooling. State-of-the-art techniques such as electrochemical machining and wire-electrical discharge machining are viewed as largely incapable of providing complex batch redistribution chambers or contours within the interiors of these dies.
What is therefore needed is an improved honeycomb extrusion die and a method of making it that are both effective and economic in practice. The die fabrication process should not involve complex machining and/or assembly processes, and the dies produced should be sufficiently robust to resist damage in an industrial environment for the continuous production of finely structured honeycomb products.